Geothermal ground source/water source heat exchange systems typically use fluid-filled closed loops of tubing buried in the ground, or submerged in a body of water, so as to either absorb heat from, or to reject heat into, the naturally occurring geothermal mass and/or water surrounding the buried or submerged fluid transport tubing. The tubing loop is extended to the surface and is then used to circulate the naturally warmed or cooled fluid to an interior air heat exchange means.
Common and older design geothermal water-source heating/cooling systems typically circulate, via a water pump, a fluid comprised of water (or water with anti-freeze) in plastic (typically polyethylene) underground geothermal tubing so as to transfer geothermal heat to or from the ground in a first heat exchange step. Via a second heat exchange step, a refrigerant heat pump system is used to transfer heat to or from the water. Finally, via a third heat exchange step, an interior air handler (typically comprised of finned tubing and a fan, as is well understood by those skilled in the art) is used to transfer heat to or from the refrigerant to heat or cool interior air space.
Newer design geothermal DX heat exchange systems, where the refrigerant fluid transport lines are placed directly in the sub-surface ground and/or water, typically circulate a refrigerant fluid, such as R-22 or the like, in sub-surface refrigerant lines, typically comprised of copper tubing, to transfer geothermal heat to or from the sub-surface elements via a first heat exchange step. DX systems only require a second heat exchange step to transfer heat to or from the interior air space, typically also by means of an interior air handler. Consequently, DX systems are generally more efficient than water-source systems due to fewer heat exchange steps and because no water pump energy expenditure is necessary. Further, since copper is a better heat conductor than most plastics, and since the refrigerant fluid circulating within the copper tubing of a DX system generally has a greater temperature differential with the surrounding ground than the water circulating within the plastic tubing of a water-source system, generally, a DX system requires less excavation and drilling and therefore has lower installation costs than a water-source system.
While most in-ground/in-water DX heat exchange designs are feasible, various improvements have been developed to enhance overall system operational efficiencies. Several such design improvements, particularly in direct expansion/direct exchange geothermal heat pump systems, are taught in U.S. Pat. No. 5,623,986 to Wiggs; in U.S. Pat. No. 5,816,314 to Wiggs, et al.; in U.S. Pat. No. 5,946,928 to Wiggs; and in U.S. Pat. No. 6,615,601 B1 to Wiggs, the disclosures of which are incorporated herein by reference. Such disclosures encompass both horizontal and vertical oriented, sub-surface, geothermal, heat exchange means using conventional refrigerants, such as R-22. The use of a refrigerant operating at higher pressures than R-22, such as R-410A, has been found to be advantageous for use in a DX system incorporating at least one of the disclosures as taught herein. R-410A is an HFC azeotropic mixture of HFC-32 and HFC-125.
DX heating/cooling systems have multiple primary objectives. The first is to provide the greatest possible operational efficiencies. This directly translates into providing the lowest possible heating/cooling operational costs, as well as other advantages, such as, for example, materially assisting in reducing peaking concerns for utility companies. A second is to operate in an environmentally safe manner via the use of environmentally safe components and fluids. A third is to provide an economically feasible installation means at the lowest possible initial cost. A fourth is to provide sub-surface installation means within the smallest surface area possible. A fifth is to increase interior comfort levels. A sixth is to increase long-term system durability, and a seventh is to facilitate ease of service and maintenance.
While typically more efficient that conventional heating/cooling systems, DX systems have experienced practical limitations associated with the relatively large surface land areas needed to accommodate the sub-surface heat exchange tubing. First generation systems using R-22 refrigerant, for example, typically require 500 square feet of land area per ton of system design capacity to accommodate a shallow (within 10 feet of the surface) matrix of multiple, distributed, copper heat exchange tubes. Early generation borehole designs still required about one 50-100 foot (maximum) depth well/borehole per ton of system design capacity, preferably spaced at least about 20 feet apart. Such requisite surface areas effectively precluded system applications in many commercial and/or high density residential applications.
While previously implemented DX systems generally achieve their primary function of heating and cooling interior space, they have exhibited some problems during the cooling mode of operation, which primarily result from an overheating of the ground surrounding the sub-surface geothermal heat exchange tubing, causing excessively high compressor head and suction pressures. Excessively high head/suction pressures result in operational efficiency losses, higher operational power draws, premature damage to system components, and correspondingly higher operating costs. Such problems are typically caused by DX systems operating in poorly conductive soils, by over-design load outdoor temperatures, by improper refrigerant charging, and/or by undersigned (“short-looped”) sub-surface heat exchange tubing. Some of the above problems, such as over-design load temperatures caused by climatic warming or the like or poorly conductive soils caused by an unforeseen drop in ground water levels, are natural and therefore unavoidable. However, many of the problems are caused by original system design error, which is primarily observed in the cooling mode of operation. When the heating and cooling loads are equal, about 30% less exposed geothermal heat transfer tubing surface area may be required in the heating mode of operation than in the cooling mode of operation.
Accordingly, it is desirable to improve upon earlier and former DX system technologies, so as to provide a means to enhance system operational efficiencies, reduce installation costs, and/or eliminate common problems encountered in the cooling mode of operation with DX system designs.